In the U.S. as well as other parts of the world, light emitting diodes (LEDs) are being used as replacements for conventional airport lights, which include approach, runway, and taxiway lights. The reasons are that colored LEDs and bright blue LEDs with white phosphor (“white LEDs”) exhibit long-lifetime, low maintenance operation and power savings. These replacement LEDs have no infrared signature.
Conventional lights tend to exhibit peak output at short-wave infrared (SWIR) wavelengths (ranging from visible to 2 microns or more) and are, therefore, better SWIR light emitters than they are visible light emitters. The output of conventional lights at long-wave infrared (LWIR) wavelengths (8-14 microns) is low because the glass lenses do not transmit LWIR energy from inside the bulb and the outside shell is not particularly hot relative to the background temperature. As a consequence, infrared EVS looks for the SWIR output. EVS instrument Model Nos. EVS 2000 and EVS 2500 manufactured by Max-Viz, Inc., the assignee of this patent application, are each implemented with a dedicated, separate SWIR camera that is processed for the lights, and the resulting processed image information is in turn overlaid or fused with LWIR “thermal background” imagery. The “All Weather Window” EVS Systems manufactured by Kollsman, Inc. and SureSight EVS Systems manufactured by CMC Electronics are each implemented with a combined SWIR and medium wave (3-5 microns) infrared (MWIR) camera configured in a single, cryocooled focal plane array (FPA).
Because the LED replacement lights have no infrared signature, detection of them by an EVS system would require use of a separate, visible (television) camera and fusion with the IR thermal imagery. U.S. Pat. Nos. 6,806,469, 6,373,055, and 6,232,602 relate to the use of such image-of-lights fusion. The Federal Aviation Administration (FAA) proposes the use of pulse width (duty cycle) modulation to control average intensity of LED (especially bright, white LED) replacement lights. Significant advantages would redound from the use of a visible camera if it could be operationally implemented with such lights. Such concepts may need to apply to co-located visible and SWIR LEDs. The SWIR LEDs would be of benefit for certain infrared EVS systems, for the reasons explained above. Moreover, there are performance benefits stemming from operation at SWIR rather than visible wavelengths, such as availability of extremely sensitive cameras, certain limited fog-penetration advantages over visible wavelengths, and reduced solar background clutter.